The present invention relates to a technique for finely adjusting an amount of deviation of scanning positions of a plurality of light beams in a primary scanning direction in an image forming apparatus such as a laser printer, or a similar apparatus in which a plurality of light beams simultaneously scan a recording medium parallel to the primary scanning direction so that plural lines are simultaneously recorded.
In order to attain high speed scanning in a conventional optical system to conduct laser beam scanning, it is necessary to increase a rotation speed of a polygonal mirror or the like, and therefore a highly accurate production engineering for air bearings, or the like, is essential. Accordingly there is problem of a high production cost.
On the other hand, as disclosed in Japanese Patent Publication Open to Public Inspection No. 158251/1979, a scanning method is widely known in which a plurality of light emitting portions of semiconductor lasers are used, and scanning is simultaneously conducted using a plurality of scanning lines. However, in order to control pitches of scanning lines, it is required to accurately adjust an interval distance between light emitting portions, and to accurately arrange them. Further, it is required to accurately conduct the angular control of the light emitting portions in the arrangement direction, and therefore, a highly accurate adjusting mechanism is essential.
Furthermore, in a scanning optical system disclosed in Japanese Patent Publication Open to Public Inspection No. 68016/1983, the pitch of scanning line in the subsidiary scanning direction is adjustable, and in an optical writing apparatus disclosed in Japanese Patent Publication Open to Public Inspection No. 50809/1988, a scanning optical path in the subsidiary scanning direction is adjusted by using 2 adjustment screws.
Further, an optical system disclosed in Japanese Patent Publication Open to Public Inspection No. 86324/1987 relates to a 2 beam laser printer in which the position adjustment of the collimator unit itself is carried out.
An apparatus to detect deviations of scanning positions of 2 light beams in the subsidiary scanning direction in an image forming apparatus is disclosed in Japanese Patent Publication Open to Public Inspection No. 228000/1995. Japanese Patent Publication Open to Public Inspection No. 248458/1995, moreover, discloses a technique in which an amount of deviation of scanning positions among a plurality of light beams is detected, and image recording is corrected according to the detected amount of deviation.
However, in all of the above technologies, in cases where the optical scanning apparatus is structured such that a plurality of lines are simultaneously recorded using plural sets of laser beams, in a plural-beam optical scanning apparatus in which laser beams (light beams) modulated according to image data are reflected by a rotating polygonal mirror and scan the recording medium so that image information is recorded, scanning positions of a plurality of laser beams are adjusted so as to conform to predetermined positions. However, these scanning positions fluctuate due to environmental change, and therefore, there is a problem in that accurate image formation can not stably be performed.
Accordingly, while the environmental change is being observed, when it goes over a predetermined range, a countermeasure in which fine adjustment is conducted again, is taken. Generally, a recording system which conducts scanning using laser beams is easily influenced by a temperature variation. Accordingly, a temperature sensor to detect temperature is provided, and a voltage outputted from the sensor corresponding to the temperature is A/D-converted by an A/D-converter and is outputted to a CPU. In the CPU, a temperature variation is determined by comparing newest inputted temperature data to the temperature at the last fine adjustment time, and when, for example, the temperature variation more than .+-.10.degree. C. is detected, the fine adjustment is conducted again. By this fine adjustment, the scanning positions of the plurality of laser beams are set within a predetermined value.
FIG. 15 is a flow chart showing a process in which adjustment of deviation is conducted by the temperature measurement. The CPU conducts temperature measurement and calculation by an output signal from a temperature sensor, not shown in the drawing, and the A/D converter, and calculates a temperature variation by comparing the newest measurement data to the preceding measurement data. It is then determined whether the temperature difference due to the detected temperature variation is within the predetermined value. When it is within the predetermined value, the temperature measurement by the temperature sensor is again conducted. However, when the temperature variation is more than the predetermined value, fine adjustment is executed, and then temperature measurement is conducted.
However, in the above fine adjustment, it can not be judged whether the temperature variation completely coincides with the amount of deviation, and the fine adjustment may therefore not be accurate. Furthermore, there is a problem in that a time lag is generated between the temperature variation and the deviation, so that it is difficult to detect the amount of deviation in real time.
On the other hand, it is disclosed in Japanese Patent Open to Public Inspection No. 270463/1994 that a plurality of optical sensors are arranged to detect the deviation of 2 beams in the primary scanning direction accurately and in real time. However, the technique disclosed in this publication relates only monochromatic image formation, and nothing is disclosed with respect to how to control the deviation of the beams in order to superimpose a plurality of colors in the color image formation. For example, when 3 color toner images are superimposed on a photoreceptor drum, it is necessary to repeat image exposure and development for each color. That is, in order to complete the color image formation, it is necessary to conduct image exposure 3 times. In this case, when correction control of the deviation of beams is conducted for each image exposure, there is a possibility that a result of calculation of an amount of deviation is different for each color, so that color doubling occurs when, finally, each color image is superimposed.